This invention relates to intraocular lenses for implantation in the anterior chamber of a human eye and more particularly to an intraocular lens having position fixation elements whose effective size is variable so as to permit use of a lens of one size in human eyes of varying size.
The implantation of an intraocular lens is a well known and widely used technique for restoring vision after cataract surgery. The natural structure of the eye furnishes a variety of locations for fixing the position of an intraocular lens in the eye. For example, an intraocular lens can be supported anteriorly of the iris, between the scleral spur and the iris, as disclosed in my U.S. Pat. Nos. 4,174,543 and 4,092,743. Alternatively, an intraocular lens can be supported posteriorly of the iris as disclosed, for example, in U.S. Pat. No. 4,159,546 to Shearing. In practice, anterior chamber lenses are the easiest to implant because the anterior chamber angle can be viewed by the surgeon while he seats the position fixation members therein. This, of course, is not the case with posterior chamber lenses, since they seat behind the iris.
Lens implantations are not only extremely difficult and delicate operations but the use of the currently available anterior chamber lenses, even by highly skilled surgeons, entails a number of disadvantages. One of these is that due to the different sizes of the eyes of different patients a sizable inventory of lenses is currently required by the surgeon to be immediately available in the operating room during the operation. Since the internal diameter of the anterior chamber angle, that is the diameter of the groove formed between the iris and the scleral spur, varies from patient to patient, (generally within a range of 11.5 thru 14.0) millimeters, the lenses currently available are manufactured in half-millimeter increments within that range which is the normal size range for human eyes. Thus lenses are currently available with diameters of 11.5, 12.0, 12.5, 13.0, 13.5 and 14.0 millimeters. Many of these lenses are furthermore available in powers varying from 14.50 thru 22.00 diopters in one diopter increments at the low and high end of the scale and in half diopter increments in the mid-range of that scale. It is very difficult with existing instrumentation to precisely measure the diameter of the anterior chamber angle in which the anterior chamber lens will be seated, prior to the time when the patient is actually in the operating room and an incision has been made. Thus, for each half-diopter increment in the aforesaid mid-range, the surgeon must have available in the operating room seven lenses of different size. It is only after the incision has been made that the most accurate known method for measuring the anterior chamber angle diameter, namely the dip-stick method, can be used.
Once the internal diameter of the anterior chamber has been measured accurately, the surgeon chooses from his inventory of differently dimensioned lenses that lens which most nearly approximates the measured dimension.
Anterior chamber intraocular lenses are known which exhibit some degree of flexibility. Usually the flexibility is for the purpose of compensating for tolerance error in the dimension of the lens, for measurement error on the part of the surgeon, for permitting some flexure in response to normal deformation of the eyeball, or, since lenses commonly are manufactured in half millimeter sizes, for allowing for differences between actual lens size and anterior chamber angle diameter should the angle or groove diameter fall somewhere between two discrete lens sizes. For example, my U.S. Pat. No. 4,092,743 describes an anterior chamber intraocular lens in which the three-point position fixation element is sufficiently flexible to compensate for small differences, up to about one-half millimeter, between the diameter of the groove in the eye in which the lens is to be implanted and the size of the lens in its undeformed condition. Also, my U.S. Pat. No. 4,147,543 discloses a four-point anterior chamber lens which can be deflected along lines drawn diagonally through its four contact points. Such deflection is stated to be in response to the "normal distortions of the eyeball."
Since the latter lenses are intended to deflect only to the extent required to compensate for normal distortions of the eyeball so as to minimize the distortion-related trauma sometimes attributable to an intraocular lens implant, a lens of proper size i.e. one which has a dimension approximating as closely as possible the measured groove dimension must be used to achieve good results. Since the lenses come in discrete sizes and since it is extremely unlikely that the diameter of the anterior chamber angle in the eye will match exactly one of those discrete sizes, the optimum lens fit is hardly ever achieved with known anterior chamber lenses.
The known anterior chamber lenses which have flexure capability, have such capability for an entirely different purpose and to a substantially different degree than is the case with the lens according to the instant invention. None of the known anterior chamber lenses, are capable of minimizing the lens inventory in the operating room to the extent of the instant invention which provides for a single anterior chamber lens adapted to fit all eyes within the normal size range.
Posterior chamber intraocular lenses, on the other hand, have for some time now been made flexible enough to fit all normal posterior chambers. Since, however, posterior chamber lenses, particularly when implanted after extra capsular cataract removal, can be seated in the capsule i.e., in a membrane which is relatively inert in that it has no nerve endings and no blood vessels, and which can therefore readily withstand punishment imposed by seating therein of springy position fixation elements, the forces exerted by, and the length of the contact areas of, the latter, are not nearly as critical in the posterior chamber capsule as in the anterior chamber. In this connection it should also be remembered that surrounding the anterior chamber angle in the vicinity of the scleral spur is the trabecular meshwork which normally pumps fluid out of the eye into Schlemms' canal, which canal, in turn, peripherially surrounds the trabecular meshwork. Any undue pressure on the anterior chamber angle in the vicinity of the scleral spur, such as may be caused by a lens implant, may interfere with the proper functioning of Schlemms' canal and may cause pain and inflammation.
While some slight interference with the functioning of the canal does not appear to be detrimental, more substantial interference as would be occasioned by anterior chamber lenses having seating portions extending over substantial peripheral regions of the anterior chamber angle and thus sealing substantial portions of the trabecular meshwork and possibly even transmitting pressure against substantial regions of Schlemms' canal, would be detrimental. By limiting such contact to regions aggregating less than 10% of the periphery of the anterior chamber angle such detrimental interference with the functioning of Schlemms' canal may be minimized.
From the foregoing it will be seen why known posterior chamber lenses have not been used and are not useful in the anterior chamber. Thus, known posterior chamber lenses, such as the Simcoe posterior chamber lens having two or three very long and very flexible haptic loops which are curved to match their respective arcs of contact with the ciliary sulcus, so as to distribute pressure evenly over as large a surface area as possible, so as to avoid focused single-point pressure, would have a deliterious effect on the functioning of Schlemms' canal and therefore should not be used in the anterior chamber. Most other known posterior chamber lenses which exhibit flex capability sufficient to operate within the size ranges specified herein, are not of three-point contact construction and therefore do not exhibit the stability which known anterior chamber lenses are able to provide. Many posterior chamber lenses including the 2- and the 3-leg Shearing-type posterior lenses, have a construction which results in their optic portion being displaced axially when the legs of the lens are squeezed from their undeformed condition into their deformed condition, i.e. their condition after the lens has been seated in the posterior chamber. Since anterior chamber lenses however are positioned between the iris and the cornea, substantial axial movement of this type cannot be tolerated since the distances to the iris on the one hand and to the cornea on the other hand are relatively small. Therefore, if the optic, after implant surgery, is too close to the cornea, normal distortions of the eyeball can result in contact by the optic with the endothelial layer of the cornea risking severe injury to the cornea. Furthermore, many of the known posterior as well as anterior chamber lenses frequently require a deflection force substantially in excess of one gram, which force will, in the implanted lens, determine the amount of pressure exerted against the tissues of the eye in which the lens is seated. The amount of pressure, will depend not only on the force but also on the area of contact between the position fixation elements of the lens and the tissue in which they are seated. Substantially greater pressures are acceptable for seating in the capsular sulcus in the posterior chamber than are acceptable for seating in the anterior chamber angle. In the lens according to the present invention the contact areas are limited and are preferably substantially point-contact. Thus, the force exerted by the position fixation elements of the lens according to the present invention is transmitted at preferably three or four (not shown) discrete points only and is not distributed over large surface areas at any of these points. A force of less than one gram has been found to show good results. If we assume a contact area of one square millimeter then the pressure will of course be less than one gram per square millimeter. Contact areas in the order of 2 or 3 square millimeters will of course even further substantially reduce the pressure on such tissues.